The Hornady Lock-n-Load O.A.L. Gauge is the secret to achieving precision in bullet depth seating. This tool makes it easier than ever to achieve a uniform seating depth, which improves the accuracy of your loaded rounds. The Hornady Lock-n-Load O.A.L. Gauge requires the use of a Modified Case, which is sold separately. Simply put, the Hornady Lock-n-Load O.A.L. Gauge helps to achieve the proper relationship between bullet seating depth and the jump to the rifling. This gauge is accurate to within 0.001 inches and is widely recognized for being the most accurate and precise tool of its kind.